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Monograph 





TECHNICAL BULLETIN NO. 24 


FLEXIBLE GLUES 
FOR 

BOOKBINDING 



By MORRIS S. KANTROWITZ, Technical Director 

ERNEST W. SPENCER, Assistant Technical Director 

1 

FREDERICK R; BLAYLOCK, Research Associate, B.M.L 

Division of Tests and Technical Control 
United States Government Printing Office 




UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1941 

Hon. Augustus E. Giegengack, Public Printer 




GOVERNMENT PRINTING OFFICE 
TECHNICAL BULLETINS 



No. 

*1. Determination of the Fiber Content of Paper. 1923. 

*2. Tentative Specifications for Bond and Ledger Papers (super¬ 
seded by No. 4). 1925. 

*3. Technical Investigations. 1927. 

*4. Proposed Specifications for Bond and Ledger Papers. 1928. 

*5. Training and Research at the Government Printing Office. 1929. 
*6. The Necessity for Research in the Printing Industry. 1929. 
*7. Analyses of Some English Bookbinding Leathers. 1929. 

8. Preliminary Report on the Determination of pH Values and 
Total Acidity in Paper. 1930. 

*9. Progress Report on Study of News Ink and Newsprint (super¬ 
seded by No. 18). 1930. 

10. Technical Specifications for Paper Users. 1930. 

*11. Progress Report on the Determination of pH Values and Total 
Acidity in Paper. 1930. 

12. A Study of Methods of Evaluation of Kraft Paper. 1931. 

*13. Second Progress Report on Study of News Ink and Newsprint 
(superseded by No. 18). 1931. 

*14. Bindery Adhesives. 1931. 

15. Standard Mimeograph Ink and Paper. 1932. 

*16. Third Progress Report on Study of News Ink and Newsprint 
(superseded by No. 18). 1932. 

17. The Evaluation of Bronze Stamping Leaf. 1933. 

18. Newsprint and News Ink (supersedes Nos. 9, 13, and 16). 

1928-33. 

19. Classification of United States Patents on Electrotyping. 1934. 

20. Rapid Methods for the Determination of Bleached and Un¬ 

bleached Fibers in Pulp and Paper. 1934. 

21. Starch-filled Book Cloth. 1934. 

22. Permanence and Durability of Paper. 1940. 

23. Tentative Condensed Classification of Printing Industry Tech¬ 

niques. 1939. 

* Out of print. 

II 


| THE UBRAAY OF CONGRESS 

JAN 7-1942 

StfVtSfON OF DOCUMENTS 







Lx V\ se. 


Foreword 


THE principal purpose of this publication 
is to make available to the bookbinding industry information 
frequently requested concerning formulas for flexible glues 
developed and used by the United States Government Printing 
Office. 

Since it is not the purpose of this bulletin to cover in general 
the field of bookbinding adhesives, nor to treat fully the subject of 
flexible glues, the reader should not be disappointed in the omission 
of technical data which he might otherwise naturally expect. 

Such a limitation in the treatment of the subject appears justi¬ 
fiable in the light of the multiplicity of adhesive formulas, and the 
myriad uses to which they must be specially adapted, when 
employed not only for bookbinding but for many other purposes. 

The relatively recent introduction of certain compounds, some¬ 
what similar to glycerin in chemical constitution and physical 
properties, favored research to find a satisfactory substitute for 
glycerin in the manufacture of bindery adhesives and printing- 
press roller compositions. 

The expediency of finding a suitable substitute for glycerin 
strongly suggested itself in that this compound might at some 
time be diverted wholly or in part to the manufacture of munitions 
of war. 

With this expediency in mind, and the desire for possibly 
increasing the flexibility of certain glue formulas by the complete 



or partial use of glycerin substitutes as plasticizers, or softening 
agents, investigational research led to this publication. 

It is our confident hope that these developments may prove as 
valuable to the bookbinding industry in general as they have been 
in particular to the Bindery Division of the United States Govern¬ 
ment Printing Office. 

THE AUTHORS. 

Division of Tests and Technical Control, 

United States Government Printing Office, 

Washington, D. C. June 1941. 


Flexible Glues for Bookbinding 


I. INTRODUCTION 


Flexible glue is prepared by cooking 
animal glue with water and a suitable plasticizer or softening agent 
until it becomes a homogeneous mixture. Glycerin is most gen¬ 
erally used as the softening agent, but, owing to marked fluctua¬ 
tions in the price of this chemical which seriously affect production 
costs, other chemical compounds have been proposed as substitutes 
and used somewhat successfully. In the preparation of flexible 
glue the principal ingredients may be combined in varying pro¬ 
portions to give compositions possessing different degrees of 
flexibility. 

The Government Printing Office, since the publication in 1931 of 
Technical Bulletin No. 14 on Bindery Adhesives, has frequently 
been requested to furnish information on the subject of animal 
glue: specifications covering its various grades; formulas for 
flexible-glue compositions; and the technique of preparing, hand¬ 
ling, and storing such compositions. These frequent requests 
have made it desirable to issue the present publication. 

With respect to the formulas given in this publication, one 
should bear in mind that they are of optimum value only when the 
specifications given for gel strength, viscosity, and acidity of the 
glue are strictly adhered to. Any variation in the quality of the 
glue employed may necessitate corresponding modifications in the 
formulas. Such modifications may not lead to satisfactory 
results, when the glue used is of an inferior quality to that rec¬ 
ommended. 

Bookbinding, like many other industries, demands a variety of 
adhesives, obtainable from both animal and vegetable sources. 
Among these may be mentioned animal glue, fish glue, casein glue, 


1 



starch paste, and dextrin gum. Since all of these substances are 
at times loosely called “glue,” it is necessary that the word “glue” 
as used in this bulletin, be specifically defined as an adhesive 
derived from animal hide, cartilage, or bone. Homogeneous mix¬ 
tures of glue, water, and suitable softening agents will be referred 
to as flexible glues. The principal softening agents used by the 
Government Printing Office in compounding flexible glues are 
glycerin, diethylene glycol, and sorbitol sirup. 


If. HISTORICAL 






The use of glue as an adhesive dates from the earliest recorded 
times. Whoever discovered that a strong adhesive could be pro¬ 
duced by cooking pieces of animal hide, or perhaps bone, in water 
has never been ascertained. Archeological discoveries indicate that 
the Egyptians more than 4,000 years ago used glue to fasten 
together pieces of wood. The practical manufacture of glue can 
be traced back directly to the reign of William III of Holland, 
where it appears to have been introduced in 1690. Shortly after¬ 
wards, about the year 1700, England began manufacturing glue 
and established it as a permanent industry. The earliest British 
patent on the subject of glue, issued in 1754, refers to a “kind of 
glue called fish glue.” Elijah Upton, the originator of the Ameri¬ 
can Glue Company of Boston, is considered by some authorities to 
have been the first person to manufacture glue in the United 
States, in the year 1808. The United States Government in 1810 
issued a compendium of manufacturers which listed six manu¬ 
facturers of glue in the State of Pennsylvania and one in the State 
of Maryland. The first patent literature mentioning the produc¬ 
tion of glue from bones appeared in the year 1814. 


III. GENERAL DESCRIPTION 


The raw materials used in the process of manufacturing glue may 
be arranged in the following order with respect to their glue-making 


2 


qualities: (1) skin or hide, (2) connective tissue, (3) cartilage, and 
(4) bone. The better glues are derived from skin or hide and the 
poorer glues from bone. This accounts for the prevalent belief 
that, in testing glue, it is important to know whether it is a hide 
glue or a bone glue. The source of a glue is not an important 
consideration when it is purchased under specifications defining 
its gel strength and viscosity. 

The National Association of Glue Manufacturers, Inc., has 
established standards for grading animal glue. The best bone 
glue, according to these standards, has a gel strength lying between 
122 grams and 149 grams, which is the same gel strength range 
as that of the poorest grade of hide glue. Therefore a glue pos¬ 
sessing this gel strength range may be either bone or hide glue, 
or a mixture of the two. Glue which has a gel strength less than 
122 grams may be classified as bone glue, while a gel strength above 
149 grams indicates the presence of hide glue. The hide glues, 
as a general rule, are more satisfactory for making flexible-glue 
compositions. Their higher gel strength and viscosity values 
result in greater flexibility of the finished product when softening 
agents are added. 


IV. METHODS OF TESTING 

Prior to 1923, the methods of testing animal glue were more 
or less cloaked in mystery, each plant having its own secret method 
of testing and grading. This secrecy resulted in constant con¬ 
fusion among users whenever attempts were made to secure glue 
of the same quality from two or more manufacturers. Under the 
designation “high grade glue” a purchaser might obtain either a 
low gel strength glue or one with a comparatively high gel strength. 
The purchaser had no means of stating his specific requirements 
other than in terms of the glue he was using. Such terms meant 
nothing to anyone except to the particular manufacturer who 
supplied that particular glue because of the numerous and varying 
systems of grading accepted by the Industry. 


3 


Bogue 2 listed 12 physical tests for glue, then in use or proposed, 
at the time of publishing his book in 1922. These were: 

1. Jelly strength or consistency. 

2. Viscosity. 

3. Melting point. 

4. Adhesive strength. 

5. Tensile strength or elasticity. 

6. Optical rotation. 

7. Swelling capacity. 

8. Rate of setting. 

9. Foam test. 

10. Grease test. 

11. Reaction (whether acid or alkaline). 

12. Appearance, odor, color, keeping qualities, etc. 

The first two of these tests, the gel strength and viscosity 
determinations, are the ones most generally used for determining 
the grade of the glue. Another test, sometimes used, determines 
the pH value, that is the degree of acidity or alkalinity of a glue. 

The gel-strength test is based upon the fact that a solution 
containing one percent or more of gelatin, when allowed to stand 
at a temperature of approximately 50 degrees F., will form a 
firm jelly. This important property is the one around which 
most of the experimental work on gelatin and glue has centered. 
If different glue solutions of identical concentration are per¬ 
mitted to chill or set, the quality of the glue will, in general, 
correspond to the consistency of the jelly formed. 

Numerous methods have been proposed for determining the 
gel strength of glue. Among the tactual methods of comparison 
the proposal of Peter Cooper in 1844 to establish arbitrary stan¬ 
dards for glue is the best known and the one most generally ac¬ 
cepted until 1923. 

He used a series of 11 types or grades of glue as a basis of com¬ 
parison. Their regular order of gel strength varied from sample 
“A Extra,” the best, to ‘‘No. 2,” the poorest. Other manu- 

2 “The Chemistry and Technology of Gelatin and Glue/’ Robert H. Bogue, 
McGraw-Hill Book Company, Inc. (1922). 


r ■ 

L 


4 



facturers adopted somewhat similar standards but with different 
designations than those suggested by Peter Cooper. It resulted 
that purchasers were mystified rather than given any definite 
knowledge of quality. 

The grading of glue by this method was accomplished by means 
of a tactual comparison of its gel strength or resiliency with that 
of the arbitrarily selected standards. The glue to be tested and 
the various grades composing the standard range of comparison 
were melted separately in definite volumes of water and allowed 
to stand for a definite period of time in a cool place. The relative 
gel strength was determined by pressing on the glues with the 
third finger of the left hand and rated according to the feeling 
of resistance to the finger pressure. Any difference in temperature 
between the sample and the standards of comparison affected the 
decision. 

The standards used for making such arbitrary comparisons were 
themselves glues which, although carefully selected, had to be 
renewed at intervals. With such crude methods of grading, the 
human factor played too important a part. Because of the falli¬ 
bility of human judgment, it is reasonable to conclude that suc¬ 
cessive standards as selected from time to time would vary among 
themselves and would eventually show considerable variations in 
gel strength from the first prepared standards. Thus, a new set of 
standards might bear slight relationship to a set previously selected. 

Other methods of determining gel strength of glue have been 
proposed. Few of these have met with wide approval because of 
their inaccuracy or because of the complicated instruments needed 
to make the test. 

The measurement of the viscosity of glue solutions prior to 1924 
was generally made with a pipette-type viscometer, or with an 
Engler viscometer. The Engler instrument met with some oppo¬ 
sition, approximately 45 minutes being required for a single deter¬ 
mination. 

In 1923, a committee of the National Association of Glue Manu¬ 
facturers investigated the various methods proposed for testing 
glue and on October 10, 1923, adopted a standard procedure 3 for 

3 Industrial and Engineering Chemistry, Vol. 16, No. 3:310, (March 1924). 


61689-41-2 


5 



testing this material. On April 10, 1930, a slight revision was made 
and in this finally adopted procedure 4 the gel strength is deter¬ 
mined by means of the Bloom gelometer. This instrument is of 
simple construction, easy to operate, and with it different labora¬ 
tories secure check results. Viscosity is measured with a stand¬ 
ardized pipette and the results are calculated in absolute units 
called millipoises so that comparisons may be made with those 
obtained by other types of viscometers. 

Viscosity and Gel Strength 

The standard procedure now in use for testing glue may be thus 
briefly described. Fifteen grams of dry glue is placed in a glass 
container of standard dimensions with 105 grams of distilled water. 
The glue is allowed to soak in a water-cooled bath at a temperature 
of 10-15° C. until it softens -thoroughly. The container is then 
removed from the cooling bath and allowed to warm up slightly 
before placing in a hot water bath, where the solution is brought 
to a temperature of 62° C. The melting and heating period is 
carefully regulated to prevent this interval from exceeding 15 
minutes. When the desired temperature is reached the sample is 
transferred to a water-jacketed viscometer of the pipette type and 
the rate of flow of a unit volume from the pipette is determined 
at 60° C. The viscosity thus obtained is expressed in millipoises. 

After the viscosity has been determined, the sample is quickly 
cooled to a temperature of approximately 45° C., and then placed 
in a constant temperature bath at 10° C. for 16 to 18 hours. This 
bath must be controlled to within plus or minus 0.1° C. At the 
end of this chilling period the gel strength is determined by means 
of a Bloom gelometer. The gel strength of glue is the weight in 
grams required to force a plunger of definite area a distance of 4 
millimeters into the gel thus prepared. 

Viscosity and gel strength values are indexes of the quality of 
glue, the best grades possessing the highest values. 

This test procedure for gel strength and viscosity applies to glue 
in the flake or ground condition but it does not give satisfactory 

4 Industrial and Engineering Chemistry, Analytical Edition, Vol. 2:348, (July 
15, 1930). 


6 



results for testing flexible glue. Because of the variable composi¬ 
tion of flexible glues no satisfactory methods of test to indicate 
quality have been developed. In testing animal glue the standard 
test procedure depends upon the presence of only one component, 
the glue itself, which varies with the grade. Three components 
are present in flexible glue and each of these influence gel strength 
and viscosity values. These components are animal glue, water, 
and a softening agent. Gel strength and viscosity values of 
flexible-glue compositions vary according to the quality and quan¬ 
tity of each of these components. Because of these variables 
which influence the test results, flexible glues of unknown com¬ 
position can best be compared by performance tests in the bindery. 
The adhesive quality of flexible glues can be controlled by mixing 
the ingredients in accordance with previously established formulas, 
using animal glue of definite gel strength and viscosity values. 

Acidity and Alkalinity 

Acidity and alkalinity of glue are measured in terms of hydrogen 
ion activity and expressed as pH values. A pH value of 7.0 is 
neutral; values lower than 7.0 representing acidity and values 
higher than 7.0 representing alkalinity. Glues having high acidity, 
that is a low pH value, absorb less water and tend to set more 
slowly than glues having low acidity, that is a high pH value. 
Glues having pH values over 7.0, representing alkalinity, tend to 
foam. Alkaline glues do not keep as well as glues which are slightly 
acid. Requirements limiting acidity and alkalinity have been 
included in the Government Printing Office specifications for glue. 

The pH value of glue solutions may be determined by color 
comparison methods, or more accurately, by electrometric methods. 

V. DEVELOPMENTAL WORK 

Before scientific control methods were introduced into the 
Government Printing Office, flexible glue was not used to any 
great extent, and that which was used was not of standard com¬ 
position. Then all adhesives were prepared in the bindery division 


7 


where they were cooked in steam-jacketed kettles. The kettles 
from which glue had been used during the day were refilled with 
glue and water during the latter part of the afternoon and main¬ 
tained at a low temperature during the night. Early on the fol¬ 
lowing morning the heat was increased sufficiently to melt the glue, 
thus preparing it for use during the day. With this cycle in opera¬ 
tion the kettles were hardly ever completely emptied, cleaned, or 
sterilized. Decomposition of the glue, with resulting loss of 
strength, frequently occurred from lack of cleanliness and over¬ 
heating. Under such conditions the best possible results were not 
derived from the grades of glue purchased. 

Prior to 1922, when the Division of Tests and Technical Control 
was established in the Government Printing Office, animal glue 
was purchased on the basis of samples submitted by bidders and 
subjected to performance tests in the Bindery to determine their 
relative merits. This method of testing was not very dependable 
because of variations in personal judgment in making the selections. 

Review of the contracts awarded annually disclosed that in 1908 
the Government Printing Office was purchasing three different 
grades of animal glue, one grade of flexible glue, and one grade of 
tablet composition. The purchase requirements for animal glue 
were very general in substance. For example, all three grades 
were to be of the best quality; one being suitable for general 
bindery use, another being quick drying and suitable for use on 
Smyth case-making machines, and a third suitable for making 
composition rollers. The flexible glue purchased was specified for 
“gluing books to be rounded and backed.” The specifications for 
the tablet composition were that “it be white and packed in 5- 
pound or 10-pound cans.” Such indefinite specifications were but 
little changed until the contracts for the fiscal year 1925 were 
awarded. 

After the organization of the Technical Division of the Govern¬ 
ment Printing Office was completed, studies were made covering: 
(1) the use of glue in the bindery; (2) the development of formulas 
for both hard and flexible glues; and (3) suitable methods for 
preparing and handling the different grades of glue needed for 
bookbinding purposes. It soon became evident that several 


8 


different formulas of flexible glue were needed to meet the varying 
needs for flexibility in bookbindings. As a result of these studies, 
the glues purchased for the fiscal year of 1925 were awarded on the 
basis of both laboratory tests and performance tests in the bindery. 

More recently the research work on flexible glues has been 
extended to include the effectiveness of using a single grade of 
glue of high gel strength and high viscosity in all formulas. As a 
result of this research, two of the three grades of glue previously 
used in these formulas were eliminated. 

Although glue of low gel strength and low viscosity is frequently 
used in commercial binderies because of low cost, this does not 
indicate that it is economical or the most satisfactory. The first 
cost of a glue is not the correct basis for evaluation. The true 
economic evaluation of a glue consists rather in the quantity of 
water it will absorb without loss of essential adhesive properties. 
A thin mixture of glue and water, with good adhesiveness, will 
work more satisfactorily than a thick mixture with less adhesive¬ 
ness. Glue is employed to bind surfaces together rather than to 
separate them. 

One should, therefore, evaluate glue on a water-glue basis rather 
than on a dry-glue basis. The best grades of glue, those with 
highest gel strengths and viscosities, will absorb more water than 
the poorer grades, and will therefore eventually cost less. 

In this connection an example of the benefits to be derived from 
the use of a high grade of glue will be interesting. For several 
years the Government Printing Office used on a Sheridan case¬ 
making machine an adhesive composed of water and glue in the 
ratio of 1:1, the glue itself having a gel strength of 160 grams. 
The quantity of water in this mixture could not be increased without 
a decided loss in working quality. In other words, the machine 
operator could not thin the mixture after placing it in the machine. 
This case-making machine, because of the drag due to the viscous 
nature of the glue-water mixture, could be operated only at a low 
speed and a considerable number of covers had to be repaired after 
each run because of insufficient adhesion of the turn ins. This 
occurred especially in hot, humid weather on buckram cover-fabrics. 
To overcome these conditions, a new case-making glue was pre- 


9 


pared with a 400-gram gel strength glue in the proportion of 2 
parts of water to 1 part of glue. Frequently the machine operator 
found it possible to add additional water to this mixture after 
placing it in the machine, at times bringing the proportion of water 
to glue as high as 4 to 1. Better adhesion of the cover turn-ins 
to the boards was obtained with such mixtures and repairing was 
sometimes reduced as much as 75 percent. The use of the thinner 
composition decreased the drag on the case-making machine, 
allowing the speed of the operation to be increased with a corre¬ 
sponding increase in production. 

This experience illustrates the fact that the initial cost should 
not be the primary consideration in evaluating a glue, and that 
some consideration should also be given to increasing the speed of 
operations and decreasing wastage. 

VI. FLEXIBLE GLUE COMPOSITIONS 

Another advantage in using glue of high gel strength and vis¬ 
cosity, especially where flexibility is desirable, is that more-flexible 
bindery compositions may be made from this grade of material. 
The percentage of glycerin or other softening agent used with 
glue largely determines the flexibility of the resulting compositions. 
Larger proportions of such softening agents may be used with 
glues of high gel strength. 

Formulas Containing Glycerin 

As a result of the change to one grade of glue for bindery work 
it became necessary to revise all glue formulas containing animal 
glue of lower gel strength and viscosity than the quality adopted 
as standard. Before revisions of the formulas were made, experi¬ 
mental testing of the various ingredients in different proportions 
was continued until the most desirable qualities of the original 
formulas were obtained. A saving in the cost of materials also 
favored the changes. 

The Government Printing Office, prior to the introduction of 
certain substitutes for glycerin, used the following formulas in 


10 


preparing flexible glues for bindery operations. The glue used in 
each formula had the properties defined by the specifications on 
page 16 of this bulletin. The figures represent proportions by 
weight. 

Beta naphthol functions as a preservative against the formation 
of bacterial molds or other decomposition usually occurring in 
organic substances such as glue or similar colloids. 

Terpineol has no effect on the working qualities of the glue but 
is added to the composition to mask unpleasant and undesirable 
odors. 

FORMULAS—SERIES 1, WITH GLYCERIN 

A-1 For general bindery use, utilizing waste roller composition: 

Percent 


Glue. 20.80 

Glycerin. 16.60 

Waste roller composition. 9.80 

Water. 52.50 

Beta naphthol. .15 

Terpineol. .15 


100.00 

8-1 For use on gathering, stitching, and covering machines: p ercen t 


Glue. 36.30 

Glycerin. 16.60 

Water. 46.80 

Beta naphthol. .15 

Terpineol. .15 


100.00 

C-1 For use on Perfect Binding Machine: Percent 

Glue. 39.90 

Glycerin. 33.30 

Water. 26.50 

Beta naphthol. -15 

Terpineol. -15 


100.00 

D-1 Tablet composition: Percent 

Glue. 26.50 

Glycerin. 26.50 


11 























Water. 46.70 

Beta naphthol. .15 

Terpineol. .15 


100.00 

D-W Tablet composition utilizing waste roller composition: p ercen t 


Glue. 17.00 

Waste roller composition. 42.50 

Water. 39.60 

Zinc Oxide. .90 


100.00 

Formulas Containing Glycerin Substitutes 

Several years ago, when a scarcity of glycerin caused marked 
fluctuations in the price of this commodity, the Government 
Printing Office instituted research to find suitable compounds which 
might serve as satisfactory substitutes for glycerin in flexible-glue 
compositions. It was foreseen that such substitutes for use in 
flexible-glue and press-roller manufacture would become necessary 
provided that glycerin should become scarce or unobtainable 
from increased demands, which could not be commercially met, 
or from restrictions placed upon its use, as in the event of war. 

The essential requirements which a substitute must meet are: 
(1) it should possess the property of glycerin to absorb moisture 
from the air and retain an appreciable portion of moisture under 
changing atmospheric conditions and (2) the cost of the substitute 
should compare favorably with that of glycerin. 

FORMULAS—SERIES 2, WITH DIETHYLENE GLYCOL 

Diethylene glycol, one of the compounds studied in connection 
with the development of suitable substitutes for glycerin, gave 
satisfactory results in flexible-glue compositions. This compound 
is a water-white, hygroscopic liquid, having a lower specific gravity 
and viscosity than glycerin. It is miscible with water in all 
proportions and is a solvent for numerous organic compounds. 
Flexible-glue compositions in which diethylene glycol is partially 
or completely substituted for glycerin are slightly thinner, but not 
sufficiently so to interfere with the working qualities of the com- 


12 











positions. The formulas, in which diethylene glycol has been 
used, are modifications of those given on page 11 of this Bulletin, 
which have been revised as follows: 


A-2 

For general bindery use: 

Percent 


Glue. 

22 60 


Diethylene glycol. 

22.10 


Water. 

55 00 


Beta naphthol. 

.15 


Terpineol. 

.15 



100.00 

B-2 

For use on gathering, stitching, and covering machines: 

Percent 


Glue. 

36.30 


Glycerin. 

8.30 


Diethylene glycol. 

8.30 


Water. 

46.80 


Beta naphthol. 

.15 


Terpineol. 

.15 



100.00 

D-2 

Tablet composition: 

Percent 


Glue. 

26.50 


Glycerin. 

13.25 


Diethylene glycol. 

13.25 


Water. 

46.70 


Beta naphthol. 

.15 


Terpineol.. 

.15 



100.00 


FORMULAS—SERIES 3, WITH SORBITOL SIRUP 

Another compound tested to determine its usefulness as a sub¬ 
stitute for glycerin in flexible-glue compositions was sorbitol, a 
hexahydric alcohol. This compound, like glycerin, belongs to the 
series of higher alcohols. Chemically speaking, it contains six 
hydroxyl groups, which is twice the number contained in glycerin. 
From their chemical structures, it is to be expected, that these two 
compounds would have somewhat similar physical properties, and 
they do. 


13 






















Sorbitol is a very effective hygroscopic agent. It has a narrower 
humectant range and therefore is less susceptible to changes in 
atmospheric conditions than glycerin. At high atmospheric humid¬ 
ities it takes on less moisture than glycerin and at low humidities 
gives up less moisture. 

During the year 1938, commercial sorbitol in sirup form, became 
available in quantities and at a price which justified an investiga¬ 
tion into its suitability as a glycerin substitute in flexible-glue 
compositions. In the pure form sorbitol is a white, crystalline 
powder, which possesses a faint, pleasant taste. Its melting point 
is 97° C. The sorbitol sirup used in our experimental work has a 
pure sorbitol content of approximately 85 percent. 

The formulas in which sorbitol sirup has been used are revisions 
of those on page 11 of this Bulletin, and are as follows: 

A-3 For general bindery use. Percent 


Glue. 22.60 

Sorbitol sirup. 20.50 

Water. 56.60 

Beta naphthol. .15 

Terpineol. .15 


100.00 

B-3 For use on gathering, stitching, and covering machines: Percent 


Glue. 36.40 

Sorbitol sirup. 16.60 

Water. 46.70 

Beta naphthol. .15 

Terpineol. .15 


100.00 

C-3 Experimental: 

Experimental work with sorbitol sirup led to the complete replacement of 
glycerin in flexible glues with the exception of formula C-l. In that formula, 
at the time of this publication, only part of the glycerin has been experimentally 
replaced and practically tested in bindery operations with satisfactory results 
following successive partial substitutions. 

It is believed that a C-glue formula will soon be developed to completely 
replace glycerin with sorbitol sirup. The present indications are that the 
percentage of softener will require some modification to compensate for the 
higher viscosity of the sorbitol sirup as compared with glycerin. Otherwise, 
there seems no good reason for not completely substituting sorbitol for glycerine 
with satisfactory results. 


14 














D-3 Tablet composition: 


Glue. 26.50 

Sorbitol sirup. 26.50 

Water. 46.70 

Beta naphthol. .15 

Terpineol. .15 


100.00 

L-3 for gluing-off large, thick books previous to rounding and backing: 

Percent 


Glue. 22.50 

Sorbitol sirup. 25.90 

Water. 51.30 

Beta naphthol. .15 

Terpineol. .15 


100.00 

Formula L-3 was developed to overcome a tendency on the part of 
large, thick books to crack open between the signatures, along the 
backbone of the book, during the rounding and backing operations. 

When sorbitol sirup is completely substituted for glycerin in 
flexible-glue formulas, the viscosity of the composition increases due 
to the fact that sorbitol has a higher viscosity than that of glycerin. 
This may be compensated by increasing the percentage of water 
present, provided one desires to retain the same viscosity as in the 
original formula. When the substitution is made without increas¬ 
ing the percentage of water, the flexible glue will set more slowly 
but will possess greater flexibility. This increased flexibility is 
especially desirable in gluing-off operations previous to rounding 
and backing, and also facilitates hand work. 

The time required for flexible glue to set and the flexibility of the 
composition may be increased or decreased by varying the relative 
percentages of glue, water, and softening agent in the composition. 
Any increase in the percentage of softening agent will in general 
decrease the rate of setting and increase flexibility. Conversely, 
any decrease in the percentage of softening agent will increase the 
rate of setting and decrease the flexibility of the composition. It 
is to be noted that different softening agents when added in equal 
proportions do not vary these qualities to an equal degree. 


15 














VII. PREPARATION AND STORAGE 


In the Government Printing Office flexible-glue compositions are 
prepared and stored in the following manner. The necessary- 
weight of cold water is placed in a large steam-jacketed cooking 
kettle, which is equipped with mechanical agitators. The dry, 
ground glue is added slowly while the agitators are operating. 
Steam is then turned into the jacket of the kettle and the glue is 
cooked until it becomes a smooth mixture. Then the softening 
agent, the beta naphthol, and the terpineol, are added and the 
cooking is continued at a temperature of approximately 150° F. 
until the mixture becomes homogeneous. It is then allowed to run 
from the kettle through wire screens to remove foreign matter and 
placed in pans greased with lard oil to solidify. 

Identification symbols cast from type-metal are placed in the 
bottoms of the pans before running in the hot glue. After solidi¬ 
fication the cakes are removed from the pans, leaving the symbol 
marking on each cake, and stored at a temperature of approxi¬ 
mately 55° F. until used. 

Prior to using, strips are cut from the cakes and melted in small, 
electrically heated, constant-temperature, glue pots which are easily 
cleaned and in which decomposition of the glue is minimized. 


VIII. APPENDIX 

Government Printing Office Specifications for Glue Making Materials 
ANIMAL GLUE 

General Requirements: 

All glue shall be clean and free from foreign matter and be foam¬ 
less. A solution of any grade shall not develop a strong or sour 
odor when kept 48 hours at a temperature of 25-35° C. Tests for 
viscosity and gel strength will be made according to the methods 
adopted by the National Association of Glue Manufacturers. Tests 
for moisture, non-foaming, and pH value will be made according to 
Federal Specification No. C-C-451. 


16 


Deliveries shall be made in paper-lined bags or paper-lined 
wooden barrels. 

Detailed Requirements: 

Viscosity—Not less than 130 nor more than 140 millipoises. 

Gel strength—Not less than 400 grams. 
pH value—Not less than 6.4 nor more than 7.0. 

GLYCERIN 

Specific Gravity—Not less than 1.250 at 25° C. 

Odor—Slight and inoffensive. 

Color—Not darker than No. 2 Union Colorimeter. 

Acidity or alkalinity—50 cubic centimeters of glycerin shall not 
require more than 0.3 cubic centimeter of normal acid or alkali 
for neutralization. 

Ash—Not more than 0.1 percent. 

Chlorine—Not more than 0.01 percent. 

SORBITOL 

Sorbitol content—Not less than 82 percent. 

Water content—Approximately 16 percent. 

Glucose—Not more than 0.3 percent. 

Specific gravity—1.31 at 25° C. 

Viscosity—Not less than 3,000 centipoises at 25° C. 

Refractive index—1.48 at 25° C. 

DIETHYLENE GLYCOL 


Color—Water white. 

Specific gravity—At 20° C.=1.117—1.120. 

Water—Not more than 0.3 percent. 

Acidity—Not more than 0.02 percent as acetic acid. 

Boiling range (760 mm.): Below 230° C.—none. 

Below 240° C.—not more than 20 percent. 
Below 250° C.—not less than 85 percent. 
Below 270° C.—not less than 95 percent. 


17 


BETA NAPHTHOL 


This Office purchases beta naphthol in powdered form under the 
following specification: 

Powdered beta naphthol, technical grade. 

TERPINEOL 

Specific gravity: at ~jyy C. =0.935-0.936. 

Refractive index: at 25° C. = 1.480-1.484. 


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